posix-clocks: Introduce clock_get_ktime() callback

The callsite in common_timer_get() has already a comment:
    /*
     * The timespec64 based conversion is suboptimal, but it's not
     * worth to implement yet another callback.
     */
    kc->clock_get(timr->it_clock, &ts64);
    now = timespec64_to_ktime(ts64);

The upcoming support for time namespaces requires to have access to:

 - The time in a task's time namespace for sys_clock_gettime()
 - The time in the root name space for common_timer_get()

That adds a valid reason to finally implement a separate callback which
returns the time in ktime_t format.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Co-developed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191112012724.250792-10-dima@arista.com

kernel/time/alarmtimer.c

@@ -663,7 +663,7 @@ static int alarm_clock_getres(const clockid_t which_clock, struct timespec64 *tp
  * @which_clock: clockid
  * @tp: timespec to fill.
  *
- * Provides the underlying alarm base time.
+ * Provides the underlying alarm base time in a tasks time namespace.
  */
 static int alarm_clock_get_timespec(clockid_t which_clock, struct timespec64 *tp)
 {
@@ -677,6 +677,22 @@ static int alarm_clock_get_timespec(clockid_t which_clock, struct timespec64 *tp
 	return 0;
 }
 
+/**
+ * alarm_clock_get_ktime - posix clock_get_ktime interface
+ * @which_clock: clockid
+ *
+ * Provides the underlying alarm base time in the root namespace.
+ */
+static ktime_t alarm_clock_get_ktime(clockid_t which_clock)
+{
+	struct alarm_base *base = &alarm_bases[clock2alarm(which_clock)];
+
+	if (!alarmtimer_get_rtcdev())
+		return -EINVAL;
+
+	return base->get_ktime();
+}
+
 /**
  * alarm_timer_create - posix timer_create interface
  * @new_timer: k_itimer pointer to manage
@@ -840,6 +856,7 @@ static int alarm_timer_nsleep(const clockid_t which_clock, int flags,
 
 const struct k_clock alarm_clock = {
 	.clock_getres		= alarm_clock_getres,
+	.clock_get_ktime	= alarm_clock_get_ktime,
 	.clock_get_timespec	= alarm_clock_get_timespec,
 	.timer_create		= alarm_timer_create,
 	.timer_set		= common_timer_set,

kernel/time/posix-timers.c

@@ -171,6 +171,11 @@ static int posix_get_realtime_timespec(clockid_t which_clock, struct timespec64
 	return 0;
 }
 
+static ktime_t posix_get_realtime_ktime(clockid_t which_clock)
+{
+	return ktime_get_real();
+}
+
 /* Set clock_realtime */
 static int posix_clock_realtime_set(const clockid_t which_clock,
 				    const struct timespec64 *tp)
@@ -193,6 +198,11 @@ static int posix_get_monotonic_timespec(clockid_t which_clock, struct timespec64
 	return 0;
 }
 
+static ktime_t posix_get_monotonic_ktime(clockid_t which_clock)
+{
+	return ktime_get();
+}
+
 /*
  * Get monotonic-raw time for posix timers
  */
@@ -228,12 +238,22 @@ static int posix_get_boottime_timespec(const clockid_t which_clock, struct times
 	return 0;
 }
 
+static ktime_t posix_get_boottime_ktime(const clockid_t which_clock)
+{
+	return ktime_get_boottime();
+}
+
 static int posix_get_tai_timespec(clockid_t which_clock, struct timespec64 *tp)
 {
 	ktime_get_clocktai_ts64(tp);
 	return 0;
 }
 
+static ktime_t posix_get_tai_ktime(clockid_t which_clock)
+{
+	return ktime_get_clocktai();
+}
+
 static int posix_get_hrtimer_res(clockid_t which_clock, struct timespec64 *tp)
 {
 	tp->tv_sec = 0;
@@ -781,7 +801,7 @@ static void common_hrtimer_arm(struct k_itimer *timr, ktime_t expires,
 	 * Posix magic: Relative CLOCK_REALTIME timers are not affected by
 	 * clock modifications, so they become CLOCK_MONOTONIC based under the
 	 * hood. See hrtimer_init(). Update timr->kclock, so the generic
-	 * functions which use timr->kclock->clock_get_timespec() work.
+	 * functions which use timr->kclock->clock_get_*() work.
 	 *
 	 * Note: it_clock stays unmodified, because the next timer_set() might
 	 * use ABSTIME, so it needs to switch back.
@@ -1262,6 +1282,7 @@ SYSCALL_DEFINE4(clock_nanosleep_time32, clockid_t, which_clock, int, flags,
 static const struct k_clock clock_realtime = {
 	.clock_getres		= posix_get_hrtimer_res,
 	.clock_get_timespec	= posix_get_realtime_timespec,
+	.clock_get_ktime	= posix_get_realtime_ktime,
 	.clock_set		= posix_clock_realtime_set,
 	.clock_adj		= posix_clock_realtime_adj,
 	.nsleep			= common_nsleep,
@@ -1280,6 +1301,7 @@ static const struct k_clock clock_realtime = {
 static const struct k_clock clock_monotonic = {
 	.clock_getres		= posix_get_hrtimer_res,
 	.clock_get_timespec	= posix_get_monotonic_timespec,
+	.clock_get_ktime	= posix_get_monotonic_ktime,
 	.nsleep			= common_nsleep,
 	.timer_create		= common_timer_create,
 	.timer_set		= common_timer_set,
@@ -1310,6 +1332,7 @@ static const struct k_clock clock_monotonic_coarse = {
 
 static const struct k_clock clock_tai = {
 	.clock_getres		= posix_get_hrtimer_res,
+	.clock_get_ktime	= posix_get_tai_ktime,
 	.clock_get_timespec	= posix_get_tai_timespec,
 	.nsleep			= common_nsleep,
 	.timer_create		= common_timer_create,
@@ -1326,6 +1349,7 @@ static const struct k_clock clock_tai = {
 
 static const struct k_clock clock_boottime = {
 	.clock_getres		= posix_get_hrtimer_res,
+	.clock_get_ktime	= posix_get_boottime_ktime,
 	.clock_get_timespec	= posix_get_boottime_timespec,
 	.nsleep			= common_nsleep,
 	.timer_create		= common_timer_create,

kernel/time/posix-timers.h

@@ -6,8 +6,11 @@ struct k_clock {
 				struct timespec64 *tp);
 	int	(*clock_set)(const clockid_t which_clock,
 			     const struct timespec64 *tp);
+	/* Returns the clock value in the current time namespace. */
 	int	(*clock_get_timespec)(const clockid_t which_clock,
 				      struct timespec64 *tp);
+	/* Returns the clock value in the root time namespace. */
+	ktime_t	(*clock_get_ktime)(const clockid_t which_clock);
 	int	(*clock_adj)(const clockid_t which_clock, struct __kernel_timex *tx);
 	int	(*timer_create)(struct k_itimer *timer);
 	int	(*nsleep)(const clockid_t which_clock, int flags,